High modulus polyethylene fiber bundles as reinforcement for brittle matrices

ABSTRACT

A process for producing bundles of high modulus polyethylene fibers for reinforcement in composites wherein the matrix is a brittle material such as cement, concrete, plaster of Paris or the like. The process involves passing high modulus polyethylene yarn through high pressure nip rolls to deform the individual filaments and to form a loosely adhering unitary mass or bundle of filaments which is then chopped into short lengths for use as fibrous reinforcement in composites. In a preferred embodiment, the yarn is twisted prior to being passed through the nip rolls.

This application is a division, of application Ser. No. 464,269, filedFeb. 7, 1983 now U.S. Pat. No. 4,483,727.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a process for forming bundles of highmodulus polyethylene fibers, and more particularly, to bundles of suchfibers which can be used as reinforcing agents for brittle matrices.

2. Description of the Prior Art

The use of various types of fibers for the reinforcement of brittlematrices is well-known in the prior art. For example, a paper entitled"Fibrous Reinforcement For Portland Cement" by S. Goldfein, "ModernPlastics" (April 1965), pages 156-159, discloses that the impactresistance and flexural strength of castings and moldings made fromPortland cement mixtures can be improved by the addition of organicfibers such as nylon, polypropylene, and polyethylene fibers. As notedby Goldfein, the high pH of the cement slurry prevents the use of glass,cotton, rayon, acetate and Dacron (trademark of E. I. du Pont de Nemours& Co. for a polyester fiber made from polyethylene terephthalate). Asnoted by Goldfein, a limiting factor was the viscosity effect of thefibers on the cement. Nylon was limited to a maximum of about 3% whereasthe other fibers could be used at quantities as high as 7%.

The difficulties in using such fibers as reinforcement are described inpart in U.S. Pat. No. 3,591,395 to Zonsveld et al. As noted therein, theuse of 3% by weight of nylon or 6% by weight of polypropylene would beeconomically prohibitive. Furthermore, the use of low deniermonofilaments chopped into short lengths entails some technicaldifficulties. Such filaments are normally marketed wound on smalldiameter spools. A deformation of such filaments is thus generatedwhich, after winding off, manifests itself as a tendency to curl. Theresulting monofilaments of short lengths are therefore difficult tohandle, since the fibers ball together and cannot be distributed evenlyin a water-hardenable mass. Unraveling of the fibers, as mentioned byGoldfein in his paper, is a cumbersome and time-consuming operationwhich is commercially unacceptable.

In an attempt to solve this problem, Zonsveld et al advocate the use of0.05 to 2% by weight of fibrous reinforcing elements formed from astretched and then fibrillated plastic film material which is preferablypolyolefin film. The Zonsveld et al patent contemplates the use ofeither continuous filaments or short segments of fibrillated plasticfilm material.

The same problem of the tendency of fibers to ball up in concreteproducts rather than being mixed uniformly is mentioned in Goldfein,U.S. Pat. No. 3,645,961. As stated therein, it is preferred that longerfibers be used since the strength increases with the length of thefibers, but that the difficulty of mixing increases at the same time.That is, as the length of the fibers is increased, there is a greatertendency of the mixture to ball.

In fact, this problem with the balling of fibers is sometimes employedto advantage to produce low density concretes which are referred to as"air-entrained" concretes and have an air content of up to about 10%.Thus, U.S. Pat. No. 3,679,445 to Howe describes the manufacture of suchconcretes, and suggests the use of fibers which are less than two incheslong and usually range in length between about 1/8 inch and 1 and 1/8inches. Included within the suggested fibers are those made from cotton,flax, rayon, Orlon (trademark of E. I. du Pont de Nemours & Co. for anacrylic fiber), nylon, Dacron (trademark of E. I. du Pont de Nemours &Co. for a polyester fiber made from polyethylene terephthalate),Terylene (trademark of Millhaven Fibers Limited for a polyester fiberbased on terephthalic acid), polyethylene, polypropylene, polyvinylchloride and glass. Although there are certain applications wherein suchair-entrained concretes are desirable, such is not the case wherestructural support is needed.

As noted in Oya et al, U.S. Pat. No. 3,865,779, the hydrophobicproperties of synthetic fibers when they are admixed with mortar, resultin their being poorly dispersed in the mortar and being liable to floaton the surface, which inevitably results in the product havingnonuniform physical properties. Accordingly, Oya et al discloses aprocess for preparing reinforcing additives to be applied to inorganiccements which are prepared by combining certain polymers, inorganicmaterials or cement, and a surfactant; mixing and melting the mixture;and then extruding it into fibers of desired shapes. However, the extrasteps involved in such a process severely limit its commercialviability.

Another method of mixing fiber reinforced concrete without the formationof fiber balls therein is disclosed in Dearlove et al, U.S. Pat. No.4,823,706. The method includes the steps of depositing a uniform layerof substantially individual concrete reinforcing fibers on an elongatedweb, coiling the web to contain the fibers, locating the coiled web inproximity to a concrete mixing device, and progressively unrolling theweb at a predetermined rate to discharge the layer of fibers therefrominto the mixture. The invention described therein is stated to be analternative to specially designed fiber feeders which separate fiberballs found in a package of fibers and slowly feed individual fibersinto a mixer or the like. However, both of these solutions entail theuse of extra steps and equipment which have a deleterious effect uponthe commercial value of the processes.

A number of patents disclose the use of fibers in various forms asreinforcement in composites which include brittle matrices. For example,Fischer et al, U.S. Pat. No. 3,533,203, describes the use of highlyelastic multifilament polypropylene strands, twisted or untwisted, i.e.,ropes or rovings, as the pretensioning element for precompressedconcrete. After tensioning and holding such strands by clamps, concreteis poured around the tensioning strands and allowed to set whilemaintaining the strands under tension.

Suzukawa, U.S. Pat. No. 3,607,685, discloses a composite buildinglaminant containing an inorganic cement reinforced with polypropylenemultifilaments.

Reineman, U.S. Pat. No. 3,922,413, and Duff, U.S. Pat. No. 3,949,144,both disclose a concrete structural member comprising a plurality ofalternate layers of epoxy resin containing concrete and fiber reinforcedepoxy resin. The fiber reinforcement may be synthetic fibers woven in abasket-like weave. It should be noted that the fiber reinforcement isused in the epoxy resin layers and not in the concrete.

Downing et al, U.S. Pat. No. 4,240,480, discloses cementitiouscomposites that may include synthetic organic polymer fibers, such aspolypropylene fibers, in various forms, for example roving orfibrillated sheet, as reinforcement. The compositions are prepared bysubjecting a mixture of hydraulic cement, fine aggregate, a selectedwater dispersable polymer and water to a homogenization process in orderto substantially reduce voidage in the product, and then curing anddrying the product. It should also be noted that in each of the examplestherein, the ingredients are mixed in a mixer and extruded before beingcured and dried. Such a process would not be suitable for forming largebatches of material.

Accordingly, a need exists for fibers in a form in which they can beused as reinforcements in composites, wherein the fibers are readilymixed with a brittle matrix without the tendency to balling and airentrapment which results in the nonuniformity found in the prior art, orthe use of expensive and complicated equipment to avoid such tendencies.

SUMMARY OF THE INVENTION

The present invention comprises the use of bundles of parallel orslightly twisted high modulus polyethylene fibers as reinforcement incomposites wherein the matrix is a brittle material such as cement,concrete, plaster of Paris or the like. A yarn made up of individualfilaments, with a modulus in excess of 300 grams per denier, isinitially passed through high pressure nip rolls to deform theindividual filaments and to form a loosely adhering unitary mass orbundle of filaments. This mas is then chopped into short lengths for useas fibrous reinforcement in composites. This process results in shortlengths of fibers wherein the bundle configuration is substantiallymaintained. In a preferred embodiment, the yarn is twisted prior tobeing passed through the nip rolls.

The bundles of the present invention are particularly useful asreinforcement when brittle matrices are employed in forming thinarchitectural panels such as cladding panels and surface-bonded masonry,and are also useful for reinforcing asbestos/concrete piping.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a typical bundle produced in accordancewith a preferred embodiment of the process of the present invention.

FIG. 2 is an enlarged perspective view of the end of a typical bundle ofthe present invention illustrating the partial fusing of the end whichoccurs as the compressed yarn is chopped into appropriate lengths.

FIG. 3 is an enlarged cross section of the bundle of FIG. 1 taken on theline 3--3.

FIG. 4 is a perspective view of a piece of concrete containing thebundles of the present invention which has been broken to illustrate howthe fibers in the bundles of the present invention become dispersed whenmixed in a brittle matrix and to illustrate the reinforcing propertiesof the bundles of the present invention when dispersed in a typicalbrittle matrix prior to curing.

DETAILED DESCRIPTION OF THE INVENTION

The process of the present invention produces discrete coherent bundlesof high modulus polyethylene fibers which are useful as reinforcingagents. The process comprises preparing highly oriented, multifilamentyarn; compressing the yarn; and then chopping the compressed yarn intoappropriate lengths. In the process of being compressed, the individualfilaments are deformed in cross section such that adjoining filamentsare also affected although the filaments are not fused together. Thisresults in bundles which are readily mixed with various types of brittlematrices, which matrices typically include a relatively high proportionof an aqueous medium.

The yarn for use in the present process can be prepared by theprocedures set forth in U.S. Pat. Nos. 3,962,205; 4,254,072; 4,268,470;and 4,287,149, all incorporated herein by reference. Those patentsdescribe a process for preparing high modulus polymeric materials andthe polymer materials produced thereby.

U.S. Pat. No. 4,254,072 issued to Capaccio et al, in particular, isdirected to a process for the production of a high modulus filament ofpolyethylene which comprises heating high density polyethylene to atemperature above its melting point, extruding the polymer to form afilament, subjecting the filament immediately after extrusion to atension under such conditions that the polymer is shaped withoutsubstantial orientation of its molecules, cooling the filament at a rateof cooling in excess of 15° C. per minute, and drawing the filament to ahigh draw ratio.

By "high density polyethylene" is meant a substantially linearhomopolymer of ethylene or a copolymer of ethylene containing at least95% by weight of ethylene having a density of from 0.85 to 1.0 g/cm³ asmeasured by the method of British Standards Specification No. 2782(1980) method 509B on a sample prepared according to British StandardSpecification No. 3412 (1966) Appendix A and annealed according toBritish Standard Specification No. 3412 (1976) Appendix B(1), such asfor example that produced by polymerizing ethylene in the presence of atransition metal catalyst. Preferred polymers have a weight averagemolecular weight of not more than 200,000.

Preferably the high density polyethylene has a weight average molecularweight of at least 50,000 and desirably a number average molecularweight in the range of 15,000 to 30,000. Desirably the polymer has aratio of weight average molecular weight (Mw) to number averagemolecular weight (Mn) of less than 8:1.

The polymer is heated to a temperature above its melting point,preferably in the range 150° to 320° C., most preferably from 190° to300° C., for example 230° to 280° C., and may be extruded at thattemperature by any suitable means through a die or spinneret.Immediately after extrusion it is subjected to a tension under suchconditions that the polymer is shaped by being drawn while hot withoutsubstantial orientation of its molecules, that is to say, the polymerretains a low degree of birefringence. Typically, the polymer has abirefringence of not more than 5×10⁻³.

Tension may be applied to the extruded polymer by a forwarding devicesuch as a forwarding jet of fluid, a roll or set of rolls, or a wind updevice. The applied tension must not be excessive and must be sufficientto give filaments having a relatively low birefringence.

After leaving the spinneret, the polymer is cooled, for example, bynatural cooling during its passage through air, or by quenching orcontact with fluid, particularly a liquid. The rate of cooling in air isfar in excess of 15° C. per minute and by quenching in a liquid, veryhigh rates of cooling may be obtained. The high rate of cooling preventsexcessive crystallization of the polymer which affects the subsequentdrawing of the spun filaments. Preferably the quenching restricts thedegree of crystallization in the filaments so that their density doesnot exceed a value of 0.96 g/cc.

The cooled polymer is drawn either immediately, as in a spin drawprocess, or may be stored in a convenient form and subsequently drawn.For example, the spun filament may be wound on a bobbin prior todrawing. In the drawing process the filament is drawn or stretched to ahigh draw ratio. In the present application, the "deformation ratio" or"draw ratio" is defined either as the ratio of the final length to theinitial length, or as the ratio of the cross sectional areas before andafter drawing.

The modulus of such a filament obtained at a high draw ratio, usuallygreater than 10:1, is primarily a function of the draw ratio, thebirefringence of spun filament having very little effect. Preferably thedraw ratio is at least 20:1.

The drawing performance of the spun filaments is also controlled by thetemperature and the speed of the drawing. Sufficient heat should besupplied to the undrawn filaments to enable them to draw withoutbreaking. Conveniently drawing may take place in a heated fluid, forexample, a jet or bath of fluid especially a liquid, such as, forexample, glycerol. Drawing may also be accomplished by bringing thefiber into contact with a heated surface such as a metal or ceramic hotshoe.

The draw temperature should never exceed a value of 130° C., otherwisethe filaments tend to melt and are flow drawn which does not result inthe filaments developing a high modulus. On the other hand, the drawtemperature should not fall below 90° C., otherwise the drawing processbecomes unrunable due to an excessive number of breakages in the threadline.

Spun filaments of polyethylene having a weight average molecular weightof not more than 200,000, a birefringence of not more than 5×10⁻³, and adensity of not more than 0.96 g/cc may be drawn at a temperature in therange of 90° C. to 130° C. in a single stage to a draw ratio in excessof 20:1 at a draw speed of at least 200 ft per minute.

While the drawing operation can be performed either as a single stageoperation or a plurality of stages, it is preferred that either a doublestage or triple stage process be used, and it is particularly preferredthat a triple stage process be used. By using a triple stage hot drawingsystem, it is possible to achieve a maximum draw ratio as high as 50:1,thereby providing a maximum modulus value of 923 grams per denier. Itappears that with increasing draw ratio, proportional increases inmodulus and tenacity and decreases in elongation are seen.

Particularly in the triple-stage drawing process, it is preferred thatthe yarn which is to be drawn be composed of individual filaments ofabout forty-five denier per filament or less. When using higher denieryarns (50-90 dpf) a reduction in the overall draw ratio is found whichis accompanied by lower physical properties. For such a triple stagedrawing process, the initial draw ratio is about 12:1 followed by stageshaving draw ratios of about 1.66:1 and 1.5:1 to produce an overall totaldraw ratio of 30.0:1.

Typical average tensile properties for the yarn utilizing the process ofthe present invention which has been drawn to a draw ratio of about 30:1are a tenacity of 8 grams per denier, an elongation of about 10% and amodulus of about 500 grams per denier. Preferably for a draw ratio ofbetween about 20:1 and about 50:1, a tenacity of between about 8 gpd andabout 12 gpd, an elongation of between about 25% and about 5%, and amodulus between about 300 gpd and about 900 gpd will be seen.

The tenacity (gpd), elongation (%), and modulus (gpd) are measured onsingle filaments at 72% relative humidity and 25° C. on an InstronTester (Instron Engineering Corporation, Canton, Mass.) using a constantextention ratio of 20% per minute with a gauge length of 1 inch andusing the standard test methods described in Section D2101-79 of theBook of ASTM Standards. The terms "tenacity", "elongation", and"modulus" are also defined therein. In particular, modulus is defined asthe ratio of the change in stress to the change in strain in the initialstraight line portion of the stress-strain curve.

The compressing of the drawn yarn may be performed by passing the yarnthrough a set of nip rolls using any of the various apparatus well knownto those skilled in the art. The exact pressure applied to the yarn bythe nip rolls is not critical, the determining factor being that thepressure must result in substantial deformation of the individualfilaments. Typical pressure applied by the nip rolls will be betweenabout 10 and about 300 psi.

While not being absolutely essential, it is preferred that the yarn betwisted prior to being compressed. Such twisting can be done by any ofthe apparatus well known to those skilled in the art. While any twistwill assist in the maintenance of discrete coherent bundles after thechopping operation, it is preferred that the yarn be twisted betweenabout one and about five turns per inch. It is particularly preferredthat a twist of about 2 and 1/2 turns per inch be applied.

After passing through the nip rolls, the compressed yarn is chopped intobundles having a length between about 0.5 and about 2.5 inches. Theparticular length used in not critical and will be chosen depending uponthe brittle matrix which is being reinforced. However, the longer thebundles, the more likely that the balling problem of the prior art willbe encountered. Accordingly, using bundles within the preferred rangewill tend to decrease this possibility.

The size of the bundles used can vary between about 200 yarn denier andabout 3,000 yarn denier, with a 1,500 denier bundle giving the bestcombination of workability and property enhancement of a brittle matrix.

When using a yarn as small as 200 yarn denier in preparing the bundlesof the present invention, the resulting sample is extremely fluffy.Accordingly, as this lower limit is approached, it becomes moredifficult to produce a bundle which is readily mixed with a brittlematrix.

The number of filaments making up the yarn used in the present processcan vary over a wide range from about 200 filaments to about 3,000filaments of between about 0.5 and about 2 denier per filament. Apreferred range is between about 700 filaments and about 3,000filaments, at approximately one denier per filament.

After chopping has been completed, the chopped yarn is in the form ofdiscrete bundles. In contrast to the prior art, such bundles are capableof being manually introduced into a cement mixer where they can beuniformly dispersed without further procedures.

At least in part the bundles are maintained in a discrete form due tothe ends of the individual filaments being partially fused by thechopping operation. When combined with the step of passing the yarnthrough the nip rolls wherein the fibers are deformed and compressed insuch a manner that the yarn almost resembles a twisted fibrillated tape,discrete coherent bundles are formed.

If care is not taken in the compression operation, the bundles will besmall bits of fluff which are fused at the end although the materialbetween the points will be open. However, this tendency is almostentirely suppressed by twisting the yarn prior to the choppingoperation.

The cross-sectional geometry of the fibers used in preparing the yarn isnot critical, with trilobal cross-sectional fibers processing comparablyto round cross-sectional fibers. The particular shape is not asimportant as the ability of the individual fibers to conform to theshape of the other fibers upon compression without leaving a largeportion of void space.

By "brittle matrix" in the present application is meant a material whichis weak in tension and particularly susceptible to breakup under impact.For example, cement and concrete crack at a very low strain of 0.04% orless. Other typical types of brittle matrices are plaster of Paris andmortar.

Loading the bundles of the present invention into a brittle matrixtypically results in a rapid increase in viscosity. For example, a fiberloading of two volume percent added to concrete resulted in a mix whichwas very stiff and unworkable and required the addition of more water toreach the desired slump (a measure of workability defined in ASTM testmethod C143). Accordingly, when using the bundles of the presentinvention, it may be necessary to modify the mix proportions, i.e.water:cement ratio and the cement:sand:aggregate ratio, to compensatefor this effect.

One of the applications in which the bundles of the present inventionare used advantageously, is in the reinforcement of thin architecturalpanels such as cladding panels and surface-bonded masonry which aretypically made from brittle matrices. The bundles are also useful forreinforcing asbestos/concrete piping.

FIG. 1 is a perspective view of a typical bundle 10 produce inaccordance with the preferred embodiment of the present invention. Thebundle seen in FIG. 1 is about one inch long and thus a twist of about2.5 turns is seen. This twist was imparted prior to the yarn beingpassed through the nip rolls, and the twist is substantially maintainedin the bundles as formed.

FIG. 2 is an enlarged view of the end of a typical bundle of the presentinvention. As can be seen in FIG. 2, the individual filaments 12 arepartially fused at the end 14 of the bundle 10 by the choppingoperation. This tends to keep the individual filaments 12 fromseparating until being mixed into a brittle matrix.

FIG. 3 is an enlarged cross section of a portion of the bundle of FIG. 1taken on the line 3--3. Individual filaments 12 are readily seen, and itcan be observed that the individual filaments 12 retain their individualidentity in spite of the compressing step.

At the same time, it can also be seen in FIG. 3 that the individualfilaments 12, which were originally round, have been deformed andflattened by the compressing step such that each filament 12 nowsubstantially conforms to the deformed shape of the surroundingfilaments 12. In fact, the bundle 10 is in the nature of a fibrillatedtape if an attempt is made to pull the bundle 10 apart.

FIG. 4 is an enlarged perspective view of a piece of concrete containingthe bundles of the present invention. The piece has been broken toillustrate how the bundles appear after being incorporated in a brittlematrix and to illustrate the reinforcing properties of the bundles whenthey have been dispersed prior to curing.

Within concrete 16 are seen numerous bundles 18 which serve to reinforcethe piece of concrete. As can be seen, the bundles 18 protrude atvarying lengths from the broken surface of the piece of concrete. As canbe further seen in FIG. 4, when the bundles 10 are mixed into a brittlematrix, such as concrete 16, the mixing action tends to separate theindividual filaments 12. The reinforcing pattern is typical of that seenwhen 2% by volume of the bundles is used.

In order to further illustrate the present invention and the advantagesthereof, the following specific example is given, it being understoodthat this example is intended to illustrate the invention but is notintended to act as a limitation on the scope of the present invention.

EXAMPLE

High density polyethylene having a number average molecular weight ofabout 22,000 and a weight average molecular weight of about 60,000(Alathon 7050, a trademark of E. I. du Pont de Nemours & Co. for apolyethylene resin) was produced using a 19 hole spinneret. The spinningconditions were a spinneret temperature of 280° C., a throughput of 15g/min., and a takeup speed of 160 m/min. A conventional pack arrangementwas used with 60/80 sand as a filtering media. The resulting filamentsas spun had a dpf of approximately 45 and a birefringence of 0.0045. Astatic air quench was used.

The spun yarn was drawn in three stages to a final draw ratio of 45:1.The intermediate draw ratios in the three stages were 12:1, 2.5:1, and1.5:1 and were accomplished by bringing the yarn into sliding contactwith hot shoes maintained at 120° C. The fiber properties of theresulting yarn were a modulus of about 900 grams per denier, a tenacityof about 10 grams per denier, and an elongation of about 8%.

The resulting yarn was twisted about 2.5 turns per inch and was then fedthrough nip rolls which applied a pressure of 60 psi. A fiber choppercut the compressed yarn into bundles of about one inch using nip rollsand a chopper manufactured by Precision Cutters, Inc., Alpha, N.J.

The resulting bundles were introduced at 2% by volume into a cementhaving a water:cement ratio of 1:3.3 and a cement:sand ratio of 3.3:1.The resulting mixture was mixed until the bundles had been relativelyuniformly dispersed, and then the mixture was formed into a claddingpanel. A comparison was made between a cladding panel having thereinforcement of the present invention and an unreinforced panel.

The panels were tested using the procedures described in the ASTM testmethod C348 to determine their flexural strength. The flexural strengthof the reinforced panel was 1,760 psi compared to a flexural strength of945 psi for the unreinforced panel.

While the invention has been described in various preferred embodiments,one skilled in the art will appreciate that various modifications,substitutions, omissions, and changes may be made without departing fromthe spirit thereof. Accordingly, it is intended that the scope of thepresent invention be limited solely by the scope of the followingclaims.

What is claimed is:
 1. A discrete, coherent bundle of high moduluspolyethylene fibers prepared by the process comprising:(a) preparinghighly oriented, multifilament yarn; (b) compressing the yarn to deformthe individual filaments in cross-section whereby a loosely adheringunitary mass of individual filaments is formed wherein each filamentsubstantially conforms to the deformed shape of the surroundingfilaments; and (c) chopping the compressed yarn into bundles ofappropriate lengths wherein there is at least partial fusion between thefiber ends during said chopping.
 2. A reinforced brittle matrixcomprising a reinforcing amount of the bundles of claim 1, substantiallyuniformly dispersed in a brittle matrix.
 3. The reinforced matrix ofclaim 2 in which the brittle matrix is concrete, cement, plaster ofParis, or mortar.
 4. The reinforced matrix of claim 2 which has beencast as a thin architectural panel.
 5. A discrete, coherent bundle ofhigh modulus polyethylene fibers of claim 1 wherein the yarn wascompressed by passing through a set of nip rolls.
 6. A reinforcedbrittle matrix comprising a reinforcing amount of the bundle of claim 5,substantially uniformly dispersed in a brittle matrix.
 7. The reinforcedmatrix of claim 6 in which the brittle matrix is concrete, cement,plaster of Paris, or mortar.
 8. The reinforced matrix of claim 6 whichhas been cast as a thin architectural panel.
 9. A discrete, coherentbundle of high modulus polyethylene fibers of claim 1 wherein the yarnwas twisted prior to being compressed.
 10. A reinforced brittle matrixcomprising a reinforcing amount of the bundle of claim 9, substantiallyuniformly dispersed in a brittle matrix.
 11. The reinforced matrix ofclaim 10 in which the brittle matrix is concrete, cement, plaster ofParis, or mortar.
 12. The reinforced matrix of claim 10 which has beencast as a thin architectural panel.
 13. A discrete, coherent bundle ofhigh modulus polyethylene fibers of claim 1 wherein the yarn was twistedbetween about one and five turns per inch prior to being compressed. 14.A reinforced brittle matrix comprising a reinforcing amount of thebundles of claim 13, substantially uniformly dispersed in a brittlematrix.
 15. The reinforced matrix of claim 14 in which the brittlematrix is concrete, cement, plaster of Paris, or mortar.
 16. Thereinforced matrix of claim 14 which has been cast as a thinarchitectural panel.